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The 13C nuclear magnetic resonance (nmr) spectra of epimers of rotenone and four 12a‐hydroxy‐analogues were examined to determine the stereochemical effect of the B/C ring fusion involving the 6a‐ and 12a‐carbon centers. Chemical shift differences between the epimeric carbon resonances of cis‐ and trans‐6a,12a‐compounds were notably larger than those of diastereoisomers derived from the same B/C ring junction stereochemistry. Results of the spectral analysis have been useful for the quantification of mixtures of epimers and for the measurement of rates of epimerization and oxygenation.

Delapril is an angiotensin converting enzyme (ACE) inhibitor, which exists as a mixture of rotational s-trans and s-cis isomers due to the rotation of the amide bond. Although these stereoisomers do interconvert by rotation about the amide bond, the rate of interconversion is slow, allowing separation of the isomers on the chromatographic time scale. The effect of the flow-rate, column temperature, pH, organic modifier, and counter-ion on the peak shape and the separation of the s-cis and s-trans conformers of delapril was investigated by HPLC. It was proved that the separation selectivity was improved by either a decrease of the flow-rate or an increase of the amount of the organic modifier or the addition of a positively charged counter ion. Delapril was then estimated by a reversed-phase liquid chromatographic method. This method was subsequently validated for specificity, linearity, accuracy and precision, according to the International Conference on Harmonization (ICH) guidelines. An NMR study indicates that the molecule exists as a mixture of rotational cis and trans isomers which confirms the results obtained by HPLC. The isomer ratio integrated from the obtained 1H NMR result was 75:25 in DMSO-d6 at 298 K, where the s-trans isomer was the major form.

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